This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-122886, filed Apr., 28, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a noncontact type signal transmission device which uses light to perform noncontact signal transmission between members whose relative positions change.
A noncontact type signal transmission device of this type is used, for example, in an X-ray computed tomography apparatus. A noncontact type signal transmission device called a slip ring made up of an annular conductive ring and a conductive brush that comes into contact with the ring is used in many conventional X-ray computed tomography apparatuses to perform continuous rotation (continuous scanning). This slip ring makes much noise and hence is not suitable for weak signals, although it is suitable for the supply of power to an X-ray tube.
Recently, X-ray computed tomography apparatuses using noncontact type signal transmission devices have become available on the market.
As shown in FIG. 1, for example, a noncontact type signal transmission device used in an X-ray computed tomography apparatus is comprised of a stationary portion 301, a rotating ring 302 located inside the stationary portion 301, a plurality of light-emitting diodes 101 discretely arranged on the outer surface of the rotating ring 302, and a plurality of photodiodes 201 discretely arranged on the inner surface of the stationary portion 301. An analog signal output from an X-ray detector is coded into a binary code, and the light-emitting diodes 101 are turned on/off in accordance with the code. This turn on/off operation is decoded through the photodiodes 201.
For the sake of descriptive convenience, a rotating coordinate system Xxe2x80x2Yxe2x80x2Zxe2x80x2 is defined, which rotates about the rotation axis of the rotating ring 302 at an angular velocity equal to the rotational movement of the rotating ring 302. Assume that the Xxe2x80x2-axis of this rotating coordinate system is the optical axis of light from the light-emitting diode 101 of interest, the Yxe2x80x2-axis is a tangent on the rotational orbit of the light-emitting diode 101 of interest, and the Zxe2x80x2-axis is an axis that is parallel to the rotation axis (orthogonal axis to the paper) and crosses the other axes at the light-emitting diode 101 of interest.
One of important parameters for the design of a noncontact type signal transmission device is the distance between each light-emitting diode 101 and a corresponding photodiode 201. As shown in FIG. 2, each photodiode 201 must be spaced apart from a corresponding light-emitting diode 101 by a certain distance so that the irradiation area from a given light-emitting diode 101 to the stationary portion 301 is seamlessly connected to (in practice, overlaps) the irradiation area from an adjacent light-emitting diode 101 to the stationary portion 301. If, however, each photodiode 201 is spaced apart from a corresponding light-emitting diode 101 too much, the amount of light received by the photodiode 201 decreases, resulting in a deterioration in sensitivity. The distance between each light-emitting diode 101 and a corresponding photodiode 201 must therefore be optimized.
In recent years, in the field of X-ray computed tomography apparatuses, techniques such as volume CT and digital rotational angiography have been put into practice. In some case, however, the amount of data handled in volume TC or the like becomes several ten or hundred times lager than the amount of data handled in single slice scanning.
To transmit such an enormous amount of data at high speed, the ON/OFF frequency of each light-emitting diode 101 must be increased to several MHz or several GHz in some cases.
If, however, the ON/OFF frequency of each light-emitting diode is increased, the amount of light emitted from the light-emitting diode decreases. Accordingly, the amount of light received by the photodiode decreases. As a consequence, a transmission error is likely to occur.
In general, the amount of light received is inversely proportional to the square of the distance between a light-emitting means and a light-receiving means. More specifically, as shown in FIG. 2, the amount of light received by the photodiode 201 decreases in inverse proportion to the square of a distance t1 between the light-emitting diode 101 and the photodiode 201. In contrast to this, the amount of light received increases as the distance t1 decreases. That is, a decrease in the amount of light received can be suppressed by brining the photodiode 201 close to the light-emitting diode 101.
If, however, each photodiode 201 is brought near a corresponding light-emitting diode 101 too much, an area appears, as shown in FIG. 3, in which the irradiation area from a given light-emitting diode 101 does not overlap that from the adjacent light-emitting diode 101. When the photodiode 201 passes through this non-overlap region, a transmission error tends to occur. However, if setting density of LED 101 is higher to eliminate the non-overlap region, a new problem of cost-up generates.
It is an object of the present invention to provide a noncontact type signal transmission device which can transmit a large amount of data at high speed with high reliability.
A noncontact type signal transmission device includes light-emitting devices and light-receiving devices. The member mounting the light-emitting devices or the light-receiving devices moves along a predetermined orbit relative to the member mounting another. Beam condensing devices are disposed between the light-emitting devices and the light-receiving devices. Each beam condensing device has a function of condensing light from the light-emitting device in a direction substantially perpendicular to the orbit. The function of each beam condensing device is to increase the amount of light received by each light-receiving device and to improve light reception sensitivity.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.